Various methods have been used in the past to extract soluble polysaccharides such as agar-agar, alginates, carrageenan, hemicellulose, and pectin from solid vegetable material. U.S. Pat. No. 3,901,873 discloses the extraction of agar from red algae by subjecting the raw algae to the action of a screw-extruder and then extracting in an autoclave in the presence of sulfuric acid. U.S. Pat. No. 1,814,981 discloses the formation of calcium alginate fibers by crushing and digesting raw kelp in a warm Na.sub.2 Co.sub.3 solution, extracting with warm water, and treating with calcium chloride. U.S. Pat. No. 3,907,770 discloses a process for extracting carrageenan from seaweed by digesting a seaweed slurry in a long column at an elevated temperature with water and an alkali metal or alkaline earth metal hydroxide. U.S. Pat. No. 3,337,526 discloses a counter-current extraction process for making arabinogalactan in which larch wood chips are driven by a worm screw through a multistage tubular extractor.
Pectin is typically hydrolyzed and extracted from citrus peel or other sources of pectin such as apple pomace. Acidified water (pH 1.5-2.5) is typically used as the extraction medium at temperatures of 60.degree. to 95.degree. C. for a time period ranging from 30 minutes to several hours. The extraction step is typically carried out in an aqueous slurry in a batch reactor with minimal agitation, followed by a separation using various combinations of centrifugation and/or filtration. The final separation step is usually a polishing filtration with a filter aid such as diatomaceous earth to produce a clear pectin extract that will produce highly transparent gels. Pectins are isolated from the extract solution by precipitation with alcohol, usually after an evaporative concentration. The isolation is followed by washing with alcohol, pressing, drying, and milling.
Other methods for extracting pectin have also been described. For example, U.S. Pat. No. 2,548,895 describes a continuous countercurrent process for extraction of pectin from citrus peel. A slurry of citrus peel is moved through an elongated trough with a U-shaped cross-section using a screw, and the acidified water flows in the opposite direction. The extract is filtered with diatomaceous earth. U.S. Pat. No. 2,586,407 discloses a process for the extraction of pectic substances from vegetable matter by (1) passing a weak acid solution through a column of vegetable matter to replace calcium and magnesium ions linked to pectic substances, (2) washing the column of vegetable matter with water to remove the acid, and (3) extracting the acid pectate substances by passing an alkaline solution through the column.
There is still a need for an improved process for extracting pectin and other soluble polysaccharides from vegetable material.